The present invention relates to benzoyl peroxide catalysts for use in curing polyester resins and various methods which enable the worker to ascertain visually the proper hardness or degree of cure of polyester or epoxy resin systems and to produce cured products therefrom which are substantially free from color.
It has been established procedure within the plastics industry to employ a pigment in the catalyst, which is admixed with a resin system to initiate curing of the latter, as a method of providing a visual indicator that a uniform distribution of the catalyst throughout the resin has been made. Improper amounts of catalyst or a non-homogeneous mixture of resin and catalyst will result in areas which do not cure or harden sufficiently to be acceptable.
Equally important in several of the industries is the time necessary for curing to take place. Polyester resins can be applied at room temperature to a substrate or can be molded in ovens and are generally totally cured within 15-20 minutes after initiation by the catalyst, while epoxy resins are slower, requiring up to about 24 hours to harden completely. In the repair industry, polyester resins are frequently employed and it is customary for the worker to tool the patched area to a semifinished contour before total curing occurs, subsequently finishing the area with sanding at about the time curing is completed. Here, obviously, it is imperative that the worker know precisely the degree of cure. If insufficient for tooling, the patch may be ruined; if totally cured, tooling becomes more difficult thereby decreasing efficiency. General experience regarding cure rates can be an asset but is not always the answer inasmuch as curing rates are also effected by temperature, humidity and even differing thicknesses of the patch. Despite recognition of the problems, there has not existed heretofore a simple, inexpensive method to determine the degree of cure or the completion thereof.
Furthermore, when a pigment is employed in the catalyst, color is imparted to the final cured product and, in some instances, this may be an advantage. However, many applications require that the product be free from color, particularly those which are white. Still other products are coated with a desired paint which lacks sufficient pigment to cover the color of the cured product in which instance the color will actually appear through the final paint coat. Disappearance or masking of the color imparted by the pigment upon curing of the resin would, therefore, greatly simplify painting and other coating operations if not totally eliminate them.
Pigments which can be suitably blended with peroxide catalysts and subsequently mixed with resin systems to indicate uniform dispersion of the catalyst in the resin and to impart color to the cured product, are disclosed in U.S. Pat. No. 3,181,991. Of course, the use of these pigments has not facilitated determination of the degree or actual completion of cure and, their presence in the cured product can be an impediment to painting operations.